Traditionally, a surfactant is blended in emulsion-type skin external preparations, such as cream and milky lotion, as an emulsifier; a surfactant is also blended in cleanser compositions, such as shampoo and body shampoo, as a cleanser. A surfactant has a hydrophilic group and a hydrophobic group in the molecule, and there are numerous surfactants depending on their combination, and there are various applications depending on the intended use. If we classify surfactants by focusing on the hydrophilic group, they can be classified into ionic surfactants, which dissociate into ions when dissolved in water, and nonionic surfactants, which do not dissociate into ions. Among these, the ionic surfactants are classified into anionic surfactants, cationic surfactants, and amphoteric surfactants depending on the charge generated by dissociation. On the other hand, if we focus on the hydrophobic group, linear or branched hydrocarbon surfactants, fluorosurfactants, silicone surfactants, etc. can be listed; however, widely used surfactants are hydrocarbon surfactants.
Anionic hydrocarbon surfactants exhibit an excellent foaming property, cleansing power, dispersing power, and emulsifying power. However, many of them have high Kraft points, and they cannot satisfactorily perform below the Kraft temperature. Among anionic hydrocarbon surfactants, most widely used sulfate-type surfactants have high foaming property. However, they may exhibit skin and eye irritation. In order to alleviate the irritation, alkyl-ether anionic surfactants, in which a polyoxyethylene group is inserted between the hydrocarbon group and the anionic group, are also used. However, the above-described problems have not been satisfactorily solved. For the reduction of skin and eye irritation, a means for achieving a high-molecular weight and a means for combining with an amphoteric surfactant are commonly known. In this case, however, there are occasions in which the surfactant capability and the texture in use are lowered. In addition, carboxylate-type surfactants are also known; however, the cleansing properties and texture in use are poor.
On the other hand, high-molecular nonionic surfactants containing a polyoxyalkylene group as the hydrophobic domain have been known. For example, Pluronic surfactants, which are triblock copolymers of ethylene oxide and propylene oxide, have been widely used because they have low irritability and low toxicity. However, it is necessary to blend a large amount of the surfactant for the fulfillment of the function, and there has been a problem in that a sticky feeling is generated as texture in use.